A mammograph normally includes a post onto which a breast-carrying plate and a moving pad are fixed. For an examination, a patient places her breast on the plate and the pad is lowered so as to compress the breast under examination and hold it in a fixed position. An X-ray tube is placed to one side of this assembly, generally above it. A cassette-carrier is placed on the opposite side and a cassette containing X-ray sensitive film is inserted therein. The X-ray tube is fixed to a bracket and the post of the bracket can tilted relative to the post of the mammograph. For stereography, an X-ray exposure is taken of the breast with the post at a given inclination on one side. The bracket carrying the X-ray tube is then tilted so that it takes up a different inclination, preferably symmetrical to the first relative to the post of the mammograph. A second X-ray exposure is then taken. After development, each of the exposures may reveal the presence of a particular disease which is to be located. In general, it is cancerous tumors that are being sought. A stereographic examination performed in this way makes it possible during subsequent stereotaxic measurement to determine the position in three dimensions of tumors detected in the breast under examination, i.e. to determine their positions relative to a three-dimensional frame of reference tied either to the cassette carrier or else to the pad, with positions being given in terms of X, Y, and Z coordinates.
FIG. 1a is a diagram of such a mammograph showing the two possible positions of the X-ray tube 1 on either side of the post 2 of the mammograph. FIG. 1b is a section on line AA in FIG. 1a. The following can be seen connected to the post 2: the breast-carrying plate 3, the moving pad 4, the cassette carrier 5, and the breast 6 under examination. During stereographic examination, two exposures 7 and 8 are taken which can be displayed on a stereotaxic apparatus 9.
The stereotaxic apparatus 9 includes cursors, and in this case there are at least two cursors I.sub.1 and I.sub.2 which are constituted by small circles surrounding crosses. These cursors can be displaced over the display plane of the apparatus 9 by means of measurement buttons 10 and 11. Control buttons 110 and 111 serve to select which one of the cursors I.sub.1 and I.sub.2 is to be moved. The buttons 10 and 11 are called "measurement" buttons because the coordinates x.sub.1,y and x.sub.2,y of the cursors are linked to the actions taken on the buttons 10 and 11. It is thus possible to place these cursors I.sub.1 and I.sub.2 on the image in each of the exposures 7 and 8 respectively of a tumor 12 contained in the breast. The X coordinates x.sub.1 and x.sub.2 and the Y coordinates y of these images can then be measured. It is then possible to deduce the X, Y, and Z coordinates of the tumor. For example: EQU Y=y EQU Z=ax.sub.1 +bx.sub.2 EQU Z=a'x.sub.1 +b'x.sub.2
In these expressions, a, b, a', b' are axis-changing coefficients that take account of the inclination of the principal ray of the X-ray tube 1 relative to the post 2, and of the distance between the tube 1 and the film 7, 8 at the moment the exposures were taken. The way in which the X, Y, and Z positions of a tumor are calculated is known, and may be as described in French patent application No. 2 248 535 filed Oct. 17, 1974 or as described in French patent application No. 2 645 286 filed Mar. 29, 1989, for example.
Once the position of a tumor has been determined, the position of the tip 13 of a needle 14 in a needle carrier 15 is determined for the purpose of taking action. If the coordinates of the tip are X.sub.a, Y.sub.a, and Z.sub.a, then the needle carrier 15 is displaced over a table 16 that carries it so that X.sub.a is made equal to X and Y.sub.a is made equal to Y, with Z.sub.a being sufficiently different from Z so that there is no danger of the tip 13 making contact with the breast during this displacement. The distance Z--Z.sub.a is the insertion depth, and it is determined in advance. For example, with a Stereotix apparatus made by General Electric CGR, France, this depth is 7 cm.
The needle 14 associated with a mammograph is a special needle: it is hollow. In conventional manner, two surgical tools can be inserted therein: a first tool referred to as "forceps" 17 is shown in the enlargement to the left of FIG. 1a, and another tool referred to as a "hook" 18 is shown in the enlargement to the right. The forceps is used for performing a biopsy, i.e. removing a sample of the tissue concerned. The hook is used for surgical guidance. In both cases, the manipulation is the same.
It consists in moving the needle carrier vertically along the predetermined insertion depth. The pad 4 is provided with an opening enabling the needle to be inserted. This makes it possible to keep the breast compressed during this operation. Under such conditions, the tip 13 of the needle comes to the position of the tumor 12. At this moment, or possibly previously, either the forceps 17 or the hook 18 is inserted into the needle. If a biopsy is to be performed, a sample is taken using the forceps 17, and the forceps is withdrawn from inside the hollow needle before withdrawing the hollow needle itself.
Otherwise, if it is desired to show a surgeon where action to be taken, a hook is slid down the hollow needle 14 and the hollow need is withdrawn leaving the hook to attach itself in the vicinity of the tumor. The hook 18 is attached to a thread 19 which then appears outside the breast and shows a surgeon where to go to remove the detected tumor.
As described above, the apparatus and the method of using it suffers from a drawback: when operating, the surgeon is obliged to follow the thread which terminates at the hook. The thread does not necessarily follow the best path for performing the operation. Which path is "best" may depend on the depth of tissue to be cut through or on the unattractive appearance of the post-operative scar. In addition, it can also happen that the surgeon cuts the thread and looses the marker showing the position of the hook.
According to the invention, this problem is solved by making the operation of inserting the needle completely independent from the structure of the apparatus. Thus, instead of providing a needle carrier that is displaceable in the X and the Y directions, with a needle that can be inserted along the Z direction, a needle carrier is provided that is displaceable in the X and Y directions, and optionally also the Z direction, and above all the needle is insertable in any direction. Under such circumstances, to solve automatically problems associated with locating the tumor relative to the tip of the needle, a needle carrier is provided which can be pointed towards an isocenter which is situated at the location of the tumor. The needle carrier is thus designed so that after the needle has been inserted through a predetermined length, its tip lies at the location of the tumor. Under such conditions, it becomes possible when performing a biopsy, or when installing a hook to select a route into the breast, giving rise to as little trauma as possible for the patient, and indeed which is less prone to error. For example, the more central zones of the breast may be reached from the periareolar zone. In other cases, the reference direction for installing the hook may coincide with the direction chosen by the surgeon for the incision. The thread or "clue" then serves as a much better guide for the operation. In a variant, it may be preferred for the marking hook to be tangential to the zone of pathology, thereby ensuring that it does not penetrate to the center thereof.